1. Field of the Invention
The present invention relates to a stereoscopic image display apparatus, and more particularly to such apparatus adapted for stereoscopic display of image information by a display device such as a television, a video display, a computer monitor, a game machine or the like and for stereoscopic observation of the image information from a predetermined observation area.
2. Related Background Art
For observing a stereoscopic image, there have been proposed, for example, a method of utilizing polarization spectacles for observing images of a parallax based on mutually different polarized states and a method of utilizing a lenticular lens for guiding images of a predetermined parallax, among the images of plural parallaxes (plural viewing points), to the eyes of the observer.
Among such systems, in the stereoscopic image display apparatus (stereoscopic display) utilizing the polarization spectacles, the polarized state of light is made different from the image for the left eye and that for the right eye and the left and right images are separated by the polarization spectacles. For obtaining different polarized states of light, a liquid crystal shutter is provided on the display to switch the polarizing state in synchronization with the field signal of the displayed image, while the observer utilizes the polarization spectacles whereby the left and right images are separated to the respective eyes on time-shared basis to enable stereoscopic observation.
In this method, the field frequency has to be selected as about 90 to 120 Hz in order to avoid flickering phenomenon. For this reason there is required a display with a high scanning frequency, and the display currently applicable for this purpose is limited to the CRT or the projection display utilizing the CRT.
On the other hand, there is also known a method of placing, on the surface of the display, a polarization control plate in which two polarizing plates with mutually orthogonal polarizing directions are alternately arranged in horizontal stripes and displaying, on the image display area of the display, an image for the left eye and another image for the right eye in alternate horizontal stripes corresponding to the pitch of the horizontally striped polarizing plates, while the observer achieves stereoscopic observation utilizing polarization spectacles with the polarizing plates of mutually orthogonal polarizing direction respectively for the left and right eyes.
In this method, since the left-eye image and the right-eye image are simultaneously displayed in horizontal stripes, the stereoscopic observation without flickering is possible even with a low scanning frequency such as in the liquid crystal display. Such method is disclosed, for example, in the U.S. Pat. Nos. 5,264,964 and 5,327,285.
In the following there will be given a further explanation on the above-explained method of stereoscopic observation, with reference to FIGS. 1 to 4. In FIG. 1, there are shown a notebook-shaped personal computer 101 serving as a stereoscopic image display apparatus, and a stereoscopic display 102 composed of a liquid crystal display provided, on the image display surface thereof, with a polarization control plate 4 in which two polarizing plates 4L, 4R with mutually perpendicular polarizing states are arranged in horizontal stripes.
In observing a two-dimensional image, the polarization spectacles 103 are not used, and each of the left and right eyes of the observer observes all the pixels of the liquid display 102 without recognizing the difference in the polarized state, so that the observer can observe the image as in the ordinary two-dimensional display.
In observing a three-dimensional image, the observer uses the polarization spectacles 103 in which the polarizing directions of the left and right eyes are mutually perpendicular, so that the left and right eyes respectively observe the pixels of different polarizing directions. Thus the stereoscopic observation is made possible by displaying, on the liquid crystal display 102, left and right parallax images corresponding to the polarizing directions of the polarizing plates 4L, 4R in the horizontal stripes of the polarization control plate 4.
In the following there will be explained, with reference to FIGS. 2A, 2B, 3 and 4, the principle of stereoscopic observation in the stereoscopic image display apparatus shown in FIG. 1. FIGS. 2A and 2B are schematic perspective views showing the configuration of a conventional stereoscopic display, wherein a liquid crystal display 1 is composed of glass substrates 2 (2a, 2b) and a pixel display unit 3 provided therebetween and consisting of liquid crystal, electrodes etc. In this drawing there are omitted two polarizing plates having mutually orthogonal polarizing directions and positioned respective in front of the front glass substrate 2a and behind the rear glass substrate 2b, and a rear light source positioned behind the glass substrates.
On the surface of the glass substrate 2a of the observation side, there is provided a polarization control plate 4, in which polarizing plates 4L, 4R, having the polarizing directions indicated by arrows, are arranged in horizontal stripes. Such plate can be fabricated for example by mechanical cutting or by photolithography.
Now, reference is made to FIG. 2B for explaining the relationship of the polarizing directions between the polarizing plates of the liquid crystal display 1 and the polarization control plate 4.
In the ordinary liquid crystal display, the polarizing plates 111, 112 positioned respectively in front of and behind the glass substrates sandwiching the pixel display unit 3 with the liquid crystal, are so provided as to have polarizing directions which are inclined by 45.degree. and are mutually perpendicular, in so-called Cross Nicole state. In the TN normally white mode, white and black are respectively displayed when a voltage is applied and not applied to the liquid crystal.
Since the light transmitted by the liquid crystal display is polarized in a direction of 45.degree. while the polarizing plates constituting the polarization control plate 4 have the horizontal and vertical polarizing directions as shown in FIG. 2B, there are only transmitted polarized components in such directions. If all the pixels of the pixel display unit 3 display white, the amounts of light transmitted by the respective polarizing plates are mutually same and, without the polarizion spectacles, all the pixels can be observed as a two-dimensional image, as in the ordinary liquid crystal display.
The pitch of the horizontal stripes constituted by the polarizing plates 4L, 4R of the polarization control plate 4 is selected equal to or slightly smaller than the width of a pixel row (L, R) corresponding to a scanning line of the pixel display unit 3 of the liquid crystal display 1.
In case of displaying a stereoscopic image, the pixel display unit 3 displays the left-eye image L and the right-eye image R alternately in every scanning line, in such a manner that the left-eye image L corresponds to a stripe of the polarizing plate 4L with the vertical polarizing direction and the right-eye image R corresponds to a stripe of the polarizing plate 4R with the horizontal polarizing direction.
The observer uses the polarization spectacles in which the left eye has a polarizing plate 4L with the vertical polarizing direction and the right eye has a polarizing plate 4R with the horizontal polarizing direction. Since each polarizing plate intercepts the polarized light of the perpendicular polarized direction, the left-eye image L and the right-eye image R are separated and observed respectively by the left and right eyes.
FIG. 3 is a lateral cross-sectional view of the stereoscopic display shown in FIGS. 2A and 2B. The pixel display unit 3 of the liquid crystal display 1 is composed of a black matrix 10, separating the pixels in the vertical direction, and pixel apertures 11, which are so provided that the polarizing plates 4L, 4R in the horizontal stripes of the polarization control plate 4 are positioned on the line between the eye 12 of the observer wearing the polarization spectacles 13 and each pixel aperture 11.
If the polarization control plate 4 can be positioned directly on the pixel display unit 3 of the liquid crystal display 1, there will not occur mutual aberration between the displayed pixels and the polarizing plates 4L, 4R regardless of the position of the observer. In practice, however, the glass substrate of the liquid crystal display has a certain thickness and the polarization control plate 4 cannot be positioned closer thereto. For this reason, depending on the height of the eyes 12 of the observer, the pixel display unit 3 becomes displaced with respect to the polarizing plates 4L, 4R so that the left and right images L, R cannot be satisfactorily separated and the stereoscopic observation cannot be obtained at a certain height of the eyes 12 of the observer.
More specifically, when the eyes 12 of the observer are at a position a1 in the vertical direction V, the polarizing plates 4L, 4R appear completely superposed with the pixel apertures 11, so that the left-eye image L and the right-eye image R are completely separated and a normal stereoscopic image can be observed.
However, if the eyes 12 of the observer move in the vertical direction to a position a4, the line connecting the pixel aperture 11 and the eyes 12 rides on the vertically adjacent polarizing plates of horizontal stripe shape on the polarization control plate 4, so that the light emerging from a same pixel aperture 11 contains mutually perpendicular polarized components.
Also when the eyes of the observer further move to a position a2, the light emerging from the same pixel aperture 11 completely passes through the adjacent polarized plate and horizontal stripe shape of the perpendicular polarizing direction, whereby the left eye only receives the right-eye image R while the right eye only receives the left-eye image L to realize a state of inverse stereoscopic observation.
A graph at the left-hand side of FIG. 3 indicates the change in the amount of polarized component, wherein the abscissa indicates the proportion of the polarized light component entering either eye, or the amount of crosstalk. When the eyes of the observer are at the position a1 where the left and right images are completely separated without crosstalk, there is only received the normal polarized component and the proportion of the polarized component is defined as 1. On the other hand, when there is only received the polarized component which is perpendicular to the normal component, there is reached a state of inverse stereoscopic observation and the proportion of the polarized component is defined as zero.
When the polarized components are mixed half and half, the proportion is defined as 0.5 or a state of half crosstalk. With the movement of the eye position from a1, the proportion of the polarized component gradually decreases with the corresponding increase of crosstalk, and reaches 0 at the position a2 or a3 where the state of completely inverse stereoscopic observation is reached.
FIG. 4 shows the stereoscopic observation areas at the optimum observation distance for the stereoscopic display 201 of the above-explained configuration, wherein 202 indicates areas (represented by solid lines) allowing normal stereoscopic observation while 203 indicates areas (represented by broken lines) providing inverse stereoscopic observation, and these areas appear periodically in the vertical direction V. As will be apparent from this drawing, the eye height allowing stereoscopic observation without crosstalk is limited to the line a1, and the stereoscopic observation area is practically limited in the vertical direction because of the generation of the crosstalk.
In case of three-dimensional display in the conventional stereoscopic display utilizing the polarization control plate as shown in FIGS. 1 to 4, there is generated an optimum observation height, allowing stereoscopic observation, in the vertical direction of the displayed image. Observation from a higher or lower position involves a gradually increasing amount of crosstalk, eventually reaching a state of inverse stereoscopic image, whereby the stereoscopic display performance is significantly deteriorated. Also for obtaining the stereoscopic image, the observer is required to maintain the eyes at the optimum observation height, whereby the observer is constantly subjected to a burden, leading to fatigue.
Also the limitation in the observation height limits the viewing position, thereby rendering the observation with plural persons quite difficult.